A generic thread whirling device according to the preamble of claim 1 is known from EP 1 985 397 A2. Generic thread whirling devices are configured to carry out the thread whirling method in which the workpiece is driven in a rotational manner about the spindle axis of the work spindle by means of an work spindle of the turning machine, whilst the workpiece extends through the opening of the whirling head, the whirling head is driven in a rotational manner about the first rotation axis by the first drive means in order to produce a cutting movement, and the workpiece is controlled by means of a control device of the turning machine in such a manner that it moves relative to the whirling head in the direction of the spindle axis, the feed speed of the workpiece relative to the whirling head in the direction of the spindle axis and the rotation speed of the workpiece about the spindle axis being adapted to each other in accordance with a thread profile to be formed.
The thread whirling of threads is particularly advantageous in the field of medical technology, in particular in the production of implants for surgery, such as, for example, self-locking bone screws having special threads. Such screws or elements having special threads are required, for example, for implants in the field of dental medicine, in spinal column implants and in the field of orthopaedic surgery as bone screws. In this instance, the great advantage of the thread whirling is that it enables workpieces to be processed from stainless and tough steel and from titanium and titanium alloys in a relatively simple manner and with the high degree of precision required.
The whirled thread profile is absolutely burr-free—an extremely important requirement for medical technology—and is distinguished by a high surface quality and shape accuracy. During the thread whirling operation, the thread can further be cut into the solid material in one operation so as to save time, whereby a high degree of measurement accuracy can be achieved. The precision is in the tolerance range of a few hundredths of a millimeter and can be achieved only by precise correspondence of the parameters of speed of the work spindle and feed motion of the whirling head relative to the work spindle.
The generic thread whirling device as a tool for thread whirling comprises, for example, the whirling head for example having a plurality of solid carbide cutters or thread turning chisels for thread whirling, which are arranged peripherally around the opening of the whirling head. For the desired thread profile, for example a specific cutter profile is calculated and specifically produced for this desired thread. The whirling head or the rotation axis thereof, in order to produce the cutting movement, is then inclined with respect to the spindle axis at the pitch angle of the thread to be cut.
Subsequently, the workpiece is pushed between the cutters of the whirling head into the opening, only one cutter being used in most cases. The whirling head rotates at high speed during the thread whirling operation and consequently determines the cutting speed, and the workpiece rotates at a low spindle speed and is located in the direction of the spindle axis during linear axis operation of the turning machine and determines the feed motion in the direction of the spindle axis, or the thread pitch. In particular, the correspondence of the feed speed and speed of the spindle influences the thread pitch of the thread.
The spindle speeds, in thread whirlers for threads for medical technology, are mostly in the range of approximately from 15 to 20 rpm, the feed speed then being intended to be adjusted in accordance with the desired thread pitch, that is to say precisely by one pitch per rotation of the spindle (in threads, the pitch characterises the lead, that is to say the spacing between two thread stages along the thread axis, or in other words the axial path which is travelled by one rotation of the thread).
In generic thread whirling devices, a drive means can be provided on the thread whirling device, by means of which drive means the rotation movement of the whirling head can be driven at high speed in order to produce the cutting movement. This drive means can be provided either as an indirect drive or as a direct drive, as advantageously taught in EP 1 985 397 A2.
However, with generic thread whirling devices there is always provision for the inclination angle of the whirling head which, in order to achieve the precision of the thread required for medical technology, has to be adjusted precisely to the desired thread pitch of the desired thread profile, to have to be adjusted in a manual manner. In order to even enable the required precision in this adjustment of the inclination angle by means of manual adjustment, it is necessary to provide complex and disadvantageously large translation mechanisms which can translate manual adjustment movements into minimal adjustment movements. Furthermore, the advantage of the extremely rapid processing of the workpiece in order to form the thread by the method of thread whirling is again invalidated by the above lengthy manual adjustment procedure.
In most cases, screws for medical technology are not mass-produced articles, but instead individually produced workpieces so that in practice, often before the processing of each individual workpiece, such a lengthy manual adjustment is required. This is even the case when, for a series of workpieces, workpieces having the same thread profile and the same thread pitch but different nominal diameters are intended to be thread whirled one after the other or even when, for a series of workpieces, workpieces having the same thread profile and the same nominal diameter but different thread pitches are intended to be thread whirled one after the other.
According to US 200810131224A1, a thread whirling head is known which can be fitted to a tool-carrying spindle of a milling processing centre and which has a thread cutter which is driven by means of the spindle drive. In this instance, the axes of the processing centre which can be travelled in order to move the spindle retaining the thread whirling head are used to control the movements of the thread whirling head relative to a workpiece which is clamped on a tool clamp in a translatory manner in three degrees of freedom and in one degree of freedom in a rotational manner with respect to a workpiece axis. In this instance, however, there is also the disadvantage that a spindle or a spindle drive of the processing centre has to be used to drive the thread whirling head and axes of the processing centre have to be used to control the thread whirling head since the thread whirling head has no separate drive means at all. In addition, such a thread whirling head is not suitable for use on a turning machine having a workpiece-carrying spindle since the spindle of a turning machine is configured to receive the workpiece (workpiece-carrying spindle in contrast to a workpiece-carrying spindle of a milling machine or a processing centre), and the thread whirling head consequently cannot be received on the spindle of the turning machine and consequently cannot be driven by means of the spindle.
With regard to these above-described disadvantages of generic thread whirling devices, an object of the present invention is to provide a thread whirling device which enables the thread whirling of medical workpieces to be configured in a more efficient and simpler manner, with the high degree of precision required in medical technology being maintained or even improved, in particular when different workpieces are processed one after the other.